This invention relates generally to mechanical valve actuators, and more particularly to those employing a stored energy component which provides nonpowered return of the valve on command or in case of a loss of power, return to a predetermined valve position.
The unit disclosed in its preferred embodiment uses an electrically operated gear motor coupled to an input shaft which in turn is directionally connected to an output shaft through a spring clutch. A torsion coil spring surrounds the input and output shafts and is connected so as to store energy generated by the drive motor.
The output shaft also utilizes a hydraulic dampening device operating by a pinion gear on the output shaft and an associated gear rack for driving an orificed piston in a hydraulic cylinder. The dampening system provides controlled operation when the actuator returns its coupled valve under the influence of torque supplied by the torsion spring.
Prior art actuators utilize torsion springs for storage of energy from the primary drive motor in connection with electro-mechanical clutches, or mechanical unidirectional couplings located between the drive motor and output shaft, such as ratchets, dog clutches, and friction clutches. These units, in particular, subject clutch components to large peak torque values on initiation and termination of the spring return cycle. In addition, an electro-mechanical version is substantially complex in that it includes an electro-magnetic winding which requires a control system of increased complexity. These devices, while satisfactory to some degree, are more expensive and somewhat less reliable.
An actuator of the electro-mechanical type is disclosed in U.S. Pat. Nos. 3,808,895 and 4,090,589 incorporated by reference herein.
The electro-mechanical, prior art actuator, utilizes a circumferential spring clutch connecting the input and output shafts. Actuation is accomplished through a releasable mechanical latch that restrains motion of the spring on the driven or output shaft. However, it has been found that the operation of this clutch has been somewhat unreliable in that it does not predictably control the actuating areas, resulting in excessive and uncontrollable wear on the mating shafts.
Other prior art clutches, such as those disclosed in U.S. Pat. Nos. 2,723,013 and 2,883,022, utilize a torsion spring surrounding the drive and driven shafts in order to effect directional drive; however, these units require a torque reversal between the driving and driven shafts, that is initiated by the driving shaft. Since in the operation of the valve actuator disclosed herein, the driving shaft essentially moves to a position against the stored energy spring and the valve actuating torque thereby holding the valve in that position for indefinite periods of time, torque reversals initiated by the driving shaft do not occur. Additionally, internal friction associated with the spring driven valve and the remainder of the drive assembly of the present actuator prevents the storage spring from releasing the driven shaft while only its lower or output end is allowed to rotate.
Therefore, it is an object of this invention to provide a unidirectional spring coupling between the rotating shafts of a valve actuator which reliably releases when a predesignated end of the spring is released to rotate while the actuator is in a "stalled" or held against a valve stop.
It is a further object of this invention to provide a unidirectional spring clutch for coupling rotating shafts where no relative motion between the shafts occurs prior to the release operation.